Thermally actuated switch



July `l, 1952 c. l. HALL ET AL THERMALLY ACTUATED SWITCH Filed May 10, 1950 5. nel;

, *F7-i? UNITEDT STATE Patented July 1, 1952 {,THERMALLY ACTUATED SWITCH l chester i. Han, scheneetaaym Johnr L. slenneger, Scotia, N. Y., assignors to General ElectricCompany, a corporation of New York Application May 10,.V 1950, Serial No. 161,088

Vl V'Ivfhis,1 invention relates lto, `electric Switches of the thermally actuated type and more particularly to switches of this type which include therein a conducting liquid such as mercury.

In gas furnaces and the like there is need for safety apparatus to detect failure of the pilot flame to prevent possible explosive damage due to the presence'of unignited fuel in the furnace in the eventofsuch a failure. In general it is desirable to.. employ electrical switching devices. actuatedlby temperatures responsive tothe pres,- enceorabsence of the pilot flame, to actuate warning devices orwtoeffect shut-off of the fuel supply through suitable valve devices. Switching devices employed for this purpose preferably have relatively yrapid response and at the same time capacity to. switch appreciable electrical loads.v In addition, the .switching elements aregpreferablyl hermetically sealed to prevent open sparking with yaccompanyingdanger of explosion. It is, of

course, essential'that 4switches employed as safety devices possess a high degree of reliability.

2y claims. (c1. zoo- 141) f taken along ythe une 2-2 of Fig. 1'; Fig. 3 is a pressure-temperature characteristic graph of the It is a general object of the present invention to provide a new and improved thermally actuated' switch which is particularly useful as la safetyfswitch; in gasffurnaces and the like, although not limited to such applications, and whicl'i4 includes the above-mentioned and other desirable features.

In accordance with the invention, a small portion of a conductive liquid, such as mercury, confined inaclosed system is subjected to relatively higl'i`or,relatively low temperatures due to the presence or absence, respectively, of a flame or the like in the vicinity of the liquid. At temperatures above the vaporization temperature of the liquid, a portion of the exposed liquid is vaporized. The vapor pressure thus created'forces the liquid intocontact with a switching electrode, thereby forming a conductive path therebetween and effecting aswitching operation. Upon cooling, thethe features vof the-,invention believed tobe novel will be pointed .out in the appended claims. In the drawing Fig., l shows an embodiment of a thermallyn actuated switch constructed in accordance with the-invention;` Fig; 2 is'. a cross-section liquid in the tube of the switch; and Fig'. 4.15 a

schematic diagram of a thermostatic control apparatus including a device of the type shown in Figs. 1 and 2.

Referring now to Fig. 1, there is shown a thermally actuated switchr I,` arranged to detect the presence or absence'of apilotflamel from a burner 3. vBurner 3 is supplied withv suitable fuel, such as gas, through a supply tube 4. `For convenience, a body member 5 is provided which serves asy the supporting structure for switch I, burnerf3 and tube 4. More particularly, an internal passageway 6 is provided in the lower portion l. ofbody 5 for conducting gas from tube 4 tofburner 3. Tube 4 is secured, as by brazing, `to portion 1 at the lower end of .passage 6 and burner 3 is secured in portion l at the upper kend of replacement of burner 3. A suitable opening 9 is provided at the lower extremity of burner ilv to admit air for combustion.v A plurality of openings I0 Vare provided at the upper extremity of burner 3 to permit escape of the combustible airgas mixture to form flame 2.

A body member ll is provided to form the main supporting structure for the elements of switch l. Member I l is secured to an upwardly extending portion l2 of body 5 by a pair of bolts I3 and nuts I4. As may be seen by reference to Fig. 2, body Il is divided. into two separate portions Ha and Hb along a surface l5, portions lla. and Hb being secured together byvpairs of bolts I6 and nuts Il. Portions Ila and' Hb may conveniently be formed of a molded material of a type suitable for the temperatures encountered in-"the vicinity of switch I.- A relatively small bulb I8, preferably formed of glassis. positioned in a cavity i9 in body ll. A capillary tube 2li'. preferably formed of anA electrically conductive material such as metal, isA secured in bulb ,I8at

the lower end thereof, thevextremity 2l -of tube.

20 being positioned at substantially the longitudinal. midpoint of bulb I8, as shown inlig. 2., Tube 20 which isgenerally U-shaped, is positionedin a recessed portion 22 of body I I sothat the other extremity 23' of tube 20 is positioned at the upper portion of body ll. A second bulb `2l,fforrned of upper extremity of bulb I8. Bulb 24 and tube 20 are completely filled with mercury 26 which extends into bulb I8 so that the level of the mercury 26 therein is between extremity 2| of tube 20 and the lower extremity 2l of electrode 25. The volume of bulb I8 above mercury 26 is charged with an inert gas 28, such as nitrogen, which may conveniently be at atmospheric pressure. The system of vessels comprising bulb I8, tube 28 and bulb 24 is sealed to prevent escape of gas therefrom. A window 29 is provided in body II to permit viewing the level of mercury 26 in bulb I8. A flexible lead 3D is connected between electrode 25 and a pair of electrically connected screws 3I of insulating terminal block 32 secured to body II. Tube 20 serves as the other electrode of the switch through contact with mercury 26. A flexible lead 33 is connected between tube 28 and electrically connected screws 34 on terminal block 32. Screws 3'I and 34 are provided as external connections of switch I. y p

The op-eration'of switch I is as follows. At relatively low temperatures mercury 26 is caused to fill tube and bul-b 24 by the pressure of gas 28. Under this condition, which exists when pilot flame 2 is not lighted, mercury 26 does not contact electrode'and switch I is open. When pilot flame 2 is lighted, bulb 24 is heated thereby to a relatively high temperature. BulbV 24 is constructed with a large surface area exposed to the flame so that heat from the name is quickly transferred to the relatively small amount of mercury therein. The volume of mercury in bulb 24 is small compared to the volume of mercury in the relatively cool tube 28 and bulb I8 so that little heat is transferred to the mercury therein.

The mercury level in bulb I 8 vs. mercury temperature in bulb 24 is shown in Fig. 3 by a curve. initially, at relatively low temperatures, when liquid mercury is present in bulb 24, expansion of the liquid mercury occurs as the temperature thereof is increased. This expansion slightly raises the level of the mercury in bulb I8 as shown by leg 43 of the graph. At the vaporization temperature 44 of the mercury under the inert gas pressure in bulb I8, vaporization of some of the mercury in bulb 24 takes place causing a substantial increase in mercury level in bulb I8. Since the volume of the inert gas 28 is reduced as a result of the increased mercury level in bulb I8, the temperature of mercury in bulb 24 must be increased slightly to drive all of the liquid mercury out of bulb 24. After all the liquid mercury has been driven out of bulb 24, conditions of superheat exist in bulb 24 represented by leg 45 of the graph and relatively little increase in pressure accompanies further increase in heat of the vaporized mercury. This feature prevents excessive pressures in glass bulb I8.

Upon removal of flame 2, the same high ratio between exposure area and the Volume of mercury in bulb 24 cools the mercury vapor, so that heat rapidly leaves the mercury causing it to condense again to liquid mercury. Upon condensation of the mercury, pressure of gas 28 in bulb I8 forces the mercury level down through tube 20, thereby breaking the contact with electrode 25, opening switch I.

'I'he construction shown is particularly adapted to obtaining a switch action which is relatively rapid, both in closing switch I due to heating bulb 24 and opening switch I due to cooling bulb 24. Since the mass of bulb 24 and liquid mercury 26 therein is relatively small, vaporization Cil occurs in a relatively short time after lighting of flame 2. It has been found that closing of switch I may be effected in two or three seconds following the lighting of flame 2.

While vaporization of mercury is described herein as occurring in a bulb 24 at the extremity of tube 20, it will be obvious that no special bulb need be provided and that if the extremity of tube 20 is sealed vaporization may equally occur in tub-e 28 itself. It is only necessary that the amount of mercury displaced due to vaporization be equal to the volume required to raise the level of mercury in bulb I 8 to provide contact with electrode 25, with reasonable overtravel to secure reliable operation. Although in the embodiment described tube -28 is shown as being U-shaped, it will be obvious that such a coniiguration is not required and that tube 20 may project downwardly or horizontally frombulb I8 since positive transfer of mercury through tube 28 is effected in both directions.

A particular feature of the construction shown comprises extending tube 20 to substantially the longitudinal midpoint of bulb I8 and introducing mercury into the closed system until bulb 24 and tube 28 are lled and the mercury extendsin bulb IB substantially halfway between the end 2i of tube 28 and the end 2T ofpelectrode 25. With this arrangement, extremity 2| Y of tube 20 is always covered by liquid mercury regardlessof the position of switch I so that there is no tendency for gas 28 to enter capillaryv tube 28 and bulb 24 even if the switch should be inverted as might occur, for example, in shipment.

While the pressure of gas 28 at normal room temperatures may conveniently be .atmospheric pressure, the operating temperature'of switch I may be readily adjusted by varying the pressure of gas 28 when the system is initially charged. Thus if the pressure of gas 28 is increased the vaporization temperature of mercury 26 is likewise increased. It will be noted that due to the leveling 01T of pressure under superheating conditions, there is, for a particular initialgas prese sure, a denite pressure limit to which the apparatus is subjected in operation and Ythus danger of failure due to bursting is minimized. A disadvantage of increasing the pressure of gas 28 above atmospheric pressure is that in case of opening of the sealed system due to breakage, for* example, there is a tendency for mercury to be forced out of the system. The switch is a failsafe device since opening of the switch occursin the event of failure of llame 2, and'when the bulb is broken the level of the mercury in bulb I8 falls also opening the switch. In addition to the above-mentioned features, the invention provides a switching apparatus having Yno moving mechanical parts and capable of switching a relatively large amount of electric power; furthermore switching takes place in a hermetically Sealed system, thereby eliminating open VVsparking.

Attention is now directed to Fig. 4 in which there is shown a thermostaticV control apparatus in which a switching deviceV I, of the general type described above and shown inFigs. 1 and`-2,-is actuated by an electrical heating element 35 in response to opening and closing of a thermo? static switch 38. Switch I, as shown in Fig. 4, is intended to comprise the same elements as shown in Figs. 1 and 2. However, for convenience switch I is represented schematically in Fig. 4 and it will be understood that certain of the elements associated with the' switch I are not showntherein. Heating element 35 is represented as a helix of resistance wire wound about bulb 24 and suitably insulated therefrom. Electrical energy for heater 35 is supplied from an electrical source 31 of alternating current, which i'srimpressed on the primary winding 38 of a transformer 39. The secondary winding 40 of transformer '39 is connected in series relation with heater `35 and the contacts il of thermostat 36. Thermostat 36, which may be any of a number of conventional thermostatic devices, is represented in Fig. 4 as having a bimetallic element 42, the bending of which, in response to temperature changes, causes opening and closing of contacts 4I.r f

If it is assumed that contacts 4l are caused to close in response to heating of element 42, electrical energy is supplied to heater 35 which, in turn, causes heating of bulb 24 and vaporization of the mercury therein. It will be obvious that the switching action of switch l is similar to the action previously described in connection with the arrangement of Figs. 1 and 2. Thus in the apparatus of Fig. 4 a relaying action' is provided since actuation of contacts 4I by heating of element 42 causes actuation of switch I. While actuation of switch l is shown as being caused lby action'of a thermostat 36, it will be obvious that other devices, such as a manual switch, may be` employed to open and close the circuit of heater 35.

heat the mercury therein, said mercury being responsive to heating of said sealed end above a predetermined temperature to vaporize therein and cause mercury from said tube to rise in said bulb and complete an electric connection between said terminals, said mercury being also responsive tc cooling of said sealed end below said predetermined temperature to condense therein to allow the pressure of said inert gas to force mercury into said tube to break the connection, said sealed end being formed to provide an increased surface for rapid transfer of heat between the wall of said tube and said mercury.

2. A thermally actuated switch comprising a frame, a vertical bulb of insulating material secured to said frame, a pair of electrodes, one of said electrodes being sealed in the upper end of said bulb and extending into the interior thereof, an electrically conductive capillary tube having the open end thereof extending into said bulb to substantially the midpoint thereof to serve as the other of said electrodes, the opposite end of said tube being sealed and of a heat conductive While wel have shown and described certain preferred embodiments of our invention it will be understood that our invention may well take other forms and We, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A thermal switch comprising a frame, a

vertical bulb of insulating material secured to said frame, an electrode sealed in the top of said bulb and extending into the interior thereof, an electrically conductive capillary tube sealed in the bottom of said bulb, the opposite end of said tube being sealed and of a heat conductive'material and projecting from said bulb, a pair of terminals insulatingly secured to said frame and respectively electrically connected to said electrode and said tube, mercury entirely lling said tube at all times and filling said bulb to a level between said tube and said electrode, an inert gas filling the remainder of said bulb, means for heating the heat conductive end of said tube to material and extending above said bulb, a quantity of mercury entirely filling said tube and illing said bulb to a predetermined level substantially midway between said electrodes, an inert gas filling the remainder of said bulb, a pair of terminals secured to and insulated from said frame and respectively electrically connected to said electrode and said tube, means for heating the heat conductive end of said tube to heat the mercury therein, said mercury being responsive to heating of said sealed end to vaporize and thereby force the mercury from said tube to flow into said bulb to raise the level of said mercury therein above the end of said electrode and compress said gas, thereby completing an electrical connection between said terminals, said mercury being also responsive to cooling of said sealed end to condense therein and allow the pressure of said inert gas to force mercury back into said tube thereby breaking said connection.

CHESTER I. HALL. JOHN L. SLONNEGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Y Name Date 1,560,119 Stoekle Nov. 3, 1925 2,150,050 Chilowsky Mar. 7, 1939 2,174,818 Brace Oct. 3, 1939 

